Only one flatfish (4)
I believe the answer is:
sole
I believe this clue is a double definition.
'only one' is the first definition.
'flatfish' is the second definition.
(sole is a kind of flatfish)
(Other definitions for sole that I've seen before include "Fish; part of foot" , "Shoe part -- lone" , "Flatfish; exclusive" , "Dover - -" , "fish > that's never less than foot in length" .)